Memory devices are typically provided as internal storage areas in the computer. The term memory identifies data storage that comes in the form of integrated circuit chips. In general, memory devices contain an array of memory cells for storing data, and row and column decoder circuits coupled to the array of memory cells for accessing the array of memory cells in response to an external address.
One type of memory is a non-volatile memory known as Flash memory. A flash memory is a type of EEPROM (electrically-erasable programmable read-only memory) that can be erased and reprogrammed in blocks. Many modern personal computers (PCs) have their BIOS stored on a flash memory chip so that it can easily be updated if necessary. Such a BIOS is sometimes called a flash BIOS. Flash memory is also popular in wireless electronic devices because it enables the manufacturer to support new communication protocols as they become standardized and to provide the ability to remotely upgrade the device for enhanced features.
A typical flash memory comprises a memory array that includes a large number of memory cells arranged in row and column fashion. Each of the memory cells includes a floating-gate field-effect transistor capable of holding a charge. The cells are usually grouped into blocks. Each of the cells within a block can be electrically programmed in a random basis by charging the floating gate. The charge can be removed from the floating gate by a block erase operation. The data in a cell is determined by the presence or absence of the charge in the floating gate.
Flash memory typically utilizes one of two basic architectures known as NOR flash and NAND flash. The designation is derived from the logic used to read the devices. In NOR flash architecture, a column of memory cells are coupled in parallel with each memory cell coupled to a bit line. In NAND flash architecture, a column of memory cells are coupled in series with only the first memory cell of the column coupled to a bit line.
To meet demands for higher capacity memories, designers continue to strive for decreasing the size of individual memory cells. However, as device size decreases, the thickness of the tunnel dielectric layer must also generally decrease. This, in turn, results in increasing risk of failure in the tunnel dielectric layer and charge leakage from the floating gate.
In addition, multi-state memory cells are becoming more prevalent, allowing designers to further increase storage density. Multi-state memory cells, such as NROM (nitride read-only memory) or SONOS (silicon oxide nitride oxide silicon) memory cells utilize localized charge trapping in a nitride layer to alter the threshold voltage of a field-effect transistor. Because the charge is localized, the cell can exhibit a first threshold voltage when read in a forward direction and a second threshold voltage when read in a reverse direction, enabling the cell to store four data values, i.e., 00, 01, 10 and 11.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for alternative memory device structures and methods of forming memory devices.